1. Technical Field
The invention relates to an active matrix substrate and a liquid crystal display (LCD) panel and in particular, relates to an active matrix substrate on a TFT substrate in which both pixel electrodes and counter electrodes are formed on the TFT substrate, and an In Plane Switching (IPS) type LCD panel with the TFT substrate having the active matrix substrate.
2. Background Art
A liquid crystal display (LCD) device is widely used as a display device of an audio visual equipment and an office automation equipment in view of such advantages of thinness, light-weight and low power consumption. This LCD device includes an LCD panel and a backlight unit which supplies backlight to the LCD panel, in which a liquid crystal is sandwiched between a TFT substrate and an opposed substrate. The opposed substrate is generally provided with a color filter and a black matrix while the TFT substrate is provided with switching elements such as TFTs (Thin Film Transistors) in a matrix pattern.
Generally, operation mode of the liquid crystal display device is classified into a TN (Twisted Nematic) type and an IPS type. In the TN type, a major axis direction of an aligned liquid crystal molecule (hereinafter, referred to an LC director) is rotated in a plane vertical to the TFT substrate, while the IPS type rotates the LC director in a plane parallel to the TFT substrate.
In the IPS type LCD panel, each pixel electrode and each counter electrode are arranged to be parallel to each other and alternately formed on the TFT substrate. The LC director molecule is changed by applying a voltage between the pixel and counter electrodes and forming an electric field parallel to the substrate surface, and thus quantity of transmitted light is controlled. In the IPS type, since the LC director rotates in a plane parallel to the substrate surface, it enables to avoid such a problem in the TN type that relation between quantity of the transmitted light and the applied voltage is greatly different when seeing from the direction of the LC director and when seeing from the normal direction to the TFT substrate. In the IPS type, therefore, a wide range of viewing angle can be obtained. (See, for example, JP-2004-280130.)
However, in the IPS type LCD device, the liquid crystal molecule rotates only in one direction, and causes a problem that coloring occurs when seeing from an oblique direction in a white display state. In order to solve this problem, the pixel electrode and the common electrode are designed to have bended configuration, respectively, in one pixel.
FIG. 8 is a plan view which typically indicates a structure of one pixel in the TFT substrate included in the IPS type LCD device of the related art.
As shown in FIG. 8, the TFT substrate of the related art includes: a gate wiring 11 which extends in the horizontal direction in the drawing sheet, a common wiring (a so-called COM wiring) 12 neighboring the gate wiring almost parallel thereto, a data wiring 13 which extends in a direction approximately orthogonal to the gate wiring 11, a TFT 16 disposed in a vicinity of a crossing point of the gate wiring 11 and the data wiring 13, a line-shaped pixel electrode 14 and a counter electrode 15 almost parallel to the data wiring 13 disposed in a region surrounded by the gate wiring 11 and the data wiring 13, and a shield common wirings 20 neighboring to both sides of the data wiring 13, shielding a leakage electric field from the data wiring 13, and having a function of the counter electrode 15.
The counter electrode 15 is connected to the common wiring 12. The pixel electrode 14 is connected to the data wiring 13 via the TFT 16. The counter electrode 15 and the pixel electrode 14 are arranged alternately with a predetermined interval and bended in order to reduce the coloring. Moreover, a domain stabilization electrode 17 which extends in the direction of the gate wiring 11 is formed at each of the bended points. A reverse-rotation prevention structure 18 that regulates the direction of rotation of the liquid crystal molecule is formed at each end of the counter electrode 15 and the pixel electrode 14.
The above-mentioned TFT substrate is manufactured by using the following steps as shown in FIG. 10.
A metal film is formed on a glass substrate (Step B1). A gate wiring 11, a gate electrode, a common wiring 12, a counter electrode 15 and a shield common wiring 20 are patterned from the metal film by using a photolithography (Step B2).
A gate insulation film and a semiconductor layer are successively formed to cover the previously patterned film (Step B3). The semiconductor layer is then patterned into a predetermined shape by using the photolithography (Step B4).
A metal film is formed and patterned into a data wiring 13, a source/drain electrode of a TFT 16 and a pixel electrode 14 by using the photolithography such that the source electrode of the TFT 16 is connected to the data wiring 13 while the drain electrode of the TFT 16 is connected to the pixel electrode 14 (Step B5).
A passivation insulating film is formed to cover a previously patterned film (Step B6). A contact hole is there formed at needed locations thereof by using the photolithography (Step B7). In this example, however, the contact hole is not shown in the drawing since the related art illustrates a structure that does not require it at the pixel area. The contact hole would be needed in a peripheral circuit area besides the pixel area.
An operation of the IPS type LCD panel will be described referring to FIG. 8. By applying an ON voltage to a selected gate wiring 11 the TFT 16 associated with the gate wiring 11 is turned on to apply a signal voltage to each pixel electrode 14 from the data wiring 13.
A common voltage (so-called COM voltage) is always applied to the counter electrode 15, and thus an electric potential difference between the pixel electrode 14 and the counter electrode 15 is accumulated and maintained in a storage capacity part (a storage 19 in FIG. 8) and a liquid crystal capacity. The storage capacity is formed by overlapping the pixel electrode 14 and the counter electrode 15 via an insulating film. The liquid crystal capacity is formed in the liquid crystal part between the pixel electrode 14 and the counter electrode 15.
The gate wiring 11 is changed to an OFF voltage, then the pixel electrode 14 is electrically disconnected from the data wiring 13, and turns into a floating state with respect to the counter electrode 15 owing to capacity coupling performed by the storage capacity and liquid crystal capacity.
Since the counter electrode 14 is always in the predetermined common potential and the pixel voltage is maintained to the signal voltage when put in the transistor ON state, the electric potential difference applied to the liquid crystal at the selected pixel is also maintained, and a directional change state of the liquid crystal molecule caused by applying the ON voltage to the selected gate wiring can be maintained.
In the IPS type LCD device, however, a horizontal or lateral electric field between the pixel electrode and the counter electrode is affected by other electric potential in the neighborhood to change the electric field strength and causes unstable display state. To improve the display state, the shield common wiring is provided aside of the data wiring. An area utilized as an opening in a pixel becomes small, and causes a problem that an aperture ratio (a ratio of the area of the opening to the area of the pixel) decreases.
In addition, since the data wiring is not an electrode of which the pixel opening is composed, the area utilized as the opening becomes smaller, and further the aperture ratio decreases.